1. Field of the Invention
The present invention relates to a method of forming a conductive pattern such as electrodes on a compound semiconductor layer, and more particularly, to a method of forming a fine electrode pattern using a lift-off process.
2. Description of the Related Art
In a technical field of an optical communication and optical recording, the demand of using a surface emitting semiconductor laser (Vertical Cavity Surface Emitting Laser Diode) has been increased because of facilitation of the two-dimensional light source array. The surface emitting semiconductor laser device is typically made of compound semiconductor layers such as GaAs, AlGaAs and so on, and generally the lift-off process is employed to form electrodes and wirings with a predetermined shape and size.
For example, Japanese Laid-Open Patent Application No. 10-154707 discloses a lift-off process. As shown in FIGS. 10A to 10E, an upper photoresist layer 15 is formed on a lower photoresist layer 13 and the upper photoresist layer 15 is insolubilized after patterning of two photoresist layers. Then the lower photoresist layer 13 is dissolved to form an undercut for the upper photoresist layer 15n. After that, a conductive film (unshown) is formed by the lift-off process. By the formation of the undercut, the needed time for the lift-off process is shortened and the generation of flashes around the conductive layer is prevented.
However, there are following problems to be resolved in forming the pattern by the lift-off process as shown in the above application. When the electrodes and/or the metal wirings pattern are formed on the compound semiconductor layer such the surface emitting semiconductor laser device, the use of an alkali developer for forming the undercut in the lower resist layer causes the contact with the surface of the compound semiconductor layers and causes them to be etched. The compound semiconductor layer, e.g. GaAs, has a surface that is chemically weak, and therefore the etching progressives clearly memorably and the life time of devices may be shortened by the surface damages caused by the etching and the performances may be affected by the damages. Furthermore, when a p-side electrode which defines an emission window for the laser light is formed the contact layer such GaAs in the surface emitting semiconductor laser, the etching of a contact layer has an adverse effect on the optical characteristics or properties of the laser light.